7.2: Absolute Dating

Relative time allows scientists to tell the story of Earth events, but does not provide specific numeric ages, and thus, the rate at which geologic processes operate. Based on Hutton’s principle of uniformitarianism, early geologists surmised geological processes work slowly and the Earth is very old. Relative dating principles were how scientists interpreted Earth history until the end of the 19th Century. Because science advances as technology advances, the discovery of radioactivity in the late 1800s provided scientists with a new scientific tool called radiometric dating. Using this new technology, they could assign specific time units, in this case years, to mineral grains within a rock. These numerical values are not dependent on comparisons with other rocks such as with relative dating, so this dating method is called absolute dating [5]. There are several types of absolute dating discussed in this section but radiometric dating is the most common and therefore is the focus on this section.

Figure \(\PageIndex{1}\): Canada’s Nuvvuagittuq Greenstone Belt may have the oldest rocks and oldest evidence life on Earth, according to recent studies.

Radioactive Decay

Atoms contain three particles: protons, neutrons, and electrons. Protons and neutrons are located in the nucleus, while electrons orbit around the nucleus. The number of protons determines which element you’re examining. For example, all atoms of carbon have six protons, all atoms of oxygen have eight protons, and all atoms of gold have 79 protons. The number of neutrons, however, is variable. An atom of an element with a different number of neutrons is an isotope of that element. For example, the isotope carbon-12 contains 6 neutrons in its nucleus, while the isotope carbon-13 has 7 neutrons.

Some isotopes are radioactive, which means they are unstable and likely to decay. This means the atom will spontaneously change from an unstable form to a stable form. There are two forms of nuclear decay that are relevant in how geologists can date rocks, shown in the table below:

Table \(\PageIndex{1}\): Types of nuclear decay

Particle	Composition	Effect on Nucleus
Alpha	2 protons, 2 neutrons	The nucleus contains two fewer protons and two fewer neutrons.
Beta	1 electron	One neutron decays to form a proton and an electron, which is emitted.

If an element decays by losing an alpha particle, it will lose 2 protons and 2 neutrons. If an atom decays by losing a beta particle, it loses just one electron.

Radioactive decay eventually results in the formation of stable daughter products. Radioactive materials decay at known rates. As time passes, the proportion of radioactive isotopes (parent isotope) will decrease and the proportion of daughter isotopes will increase. A rock with a relatively high proportion of radioactive isotopes is probably very young, while a rock with a high proportion of daughter products is probably very old.

Scientists measure the rate of radioactive decay with a unit called half-life. The half-life of a radioactive substance is the amount of time, on average, it takes for half of the parent isotope atoms to decay to daughter isotope atoms. For example, imagine a radioactive substance with a half-life of one year. When a rock is formed, it contains a certain number of radioactive parent isotope atoms. After one year (one half-life), half of the parent isotope atoms have decayed to form stable daughter products, and 50% of the radioactive parent isotope atoms remain. After another year (two half-lives), half of the remaining radioactive parent atoms have decayed, and 25% of the original radioactive parent atoms remain. After the third year (three half-lives), 12.5% of the original radioactive parent atoms remain. After four years (four half-lives), 6.25% of the original radioactive parent atoms remain, and after 5 years (five half-lives), only 3.125% of the original radioactive parent atoms remain.

If you find a rock whose radioactive material has a half-life of one year and measure 3.125% radioactive parent atoms and 96.875% daughter atoms, you can assume that the substance is 5 years old. The decay of radioactive materials is shown in the following graph. For example, a rock with 75% of the radioactive parent atoms remaining is about 0.4 years old.

Figure \(\PageIndex{2}\): Decay of an imaginary radioactive substance with a half-life of one year. Note that in this example, the age also represents the number of half-lives that have elapsed. (By Kurt Rosenkrantz; CC BY-SA 3.0 via Wikimedia Commons.)

Radiometric Dating of Rocks

In the process of radiometric dating, several isotopes are used to date rocks and other materials. Using several different isotopes helps scientists to check the accuracy of the ages that they calculate.

Carbon Dating

Earth’s atmosphere contains three isotopes of carbon. Carbon-12 (¹²C) is stable and accounts for 98.9% of atmospheric carbon. Carbon-13 (¹³C) is also stable and accounts for 1.1% of atmospheric carbon. Carbon-14 (¹⁴C) is radioactive and is found in tiny amounts. Carbon-14 is produced naturally in the atmosphere when cosmic rays interact with nitrogen atoms. The amount of carbon-14 produced in the atmosphere at any particular time has been relatively stable through time.

Radioactive ¹⁴C decays to stable nitrogen-14 (¹⁴N) by releasing a beta particle. The nitrogen atoms are lost to the atmosphere, but the amount of ¹⁴C decay can be estimated by measuring the proportion of radioactive ¹⁴C to stable ¹²C. As a substance ages, the relative amount of ¹⁴C decreases.

Carbon is removed from the atmosphere by plants during the process of photosynthesis. Animals consume this carbon when they eat plants or other animals that have eaten plants. When an organism dies, the radiocarbon clock starts ticking as the ¹⁴C decays back to ¹⁴N. Therefore carbon-14 dating can be used to date plant and animal remains. Examples include timbers from an old building, bones, or ashes from a fire pit. Carbon dating can be effectively used to find the age of materials between 100 and 50,000 years old.

Potassium-Argon Dating

Potassium-40 (⁴⁰K) decays to argon-40 (⁴⁰Ar) with a half-life of 1.26 billion years. Because argon is a gas, it can escape from molten magma or lava. Therefore any argon that is found in a crystal probably formed as a result of the decay of potassium-40. Measuring the ratio of ⁴⁰K to ⁴⁰Ar will yield a good estimate of the age of the sample.

Potassium is a common element found in many minerals such as feldspar, mica, and amphibole. Potassium-argon dating can be used to date igneous rocks from 100,000 years to over a billion years old. Because it can be used to date geologically young materials, the technique has been useful in estimating the age of deposits containing the bones of human ancestors.

Uranium-Lead Dating

Two isotopes of uranium are used for radiometric dating. Uranium-238 (²³⁸U) decays to form lead-206 (²⁰⁶Pb) with a half-life of 4.47 billion years. Uranium-235 (²³⁵U) decays to form lead-207 (²⁰⁷Pb) with a half-life of 704 million years.

Uranium-lead dating is usually performed on crystals of the mineral zircon. When zircon forms in an igneous rock, the crystals readily accept atoms of uranium but reject atoms of lead. Therefore, if any lead is found in a zircon crystal, it can be assumed that it was produced from the decay of uranium. Zircon is resistant to weathering which makes it useful for dating geological events in ancient rocks. During metamorphic events, zircon crystals may form multiple crystal layers, with each layer recording the isotopic age of an event, thus tracing the progress of the several metamorphic events [16].

Figure \(\PageIndex{3}\): Zircon crystal. (By Eurico Zimbres, Tom Epaminondas; CC BY-SA 2.0 BR via Wikimedia Commons.)

Geologists have used zircon grains to do some amazing studies that illustrate how scientific conclusions can change with technological advancements. Zircon crystals from Western Australia that formed when the crust first differentiated from the mantle 4.4 billion years ago have been determined to be the oldest known rocks [6]. The zircon grains were incorporated into metasedimentary host rocks, sedimentary rocks showing signs of having undergone partial metamorphism. The host rocks were not very old but the embedded zircon grains were created 4.4 billion years ago and survived the subsequent processes of weathering, erosion, deposition, and metamorphism. From other properties of the zircon crystals, researchers concluded that not only were continental rocks exposed above sea level but also that conditions on the early Earth were cool enough for liquid water to exist on the surface. The presence of liquid water allowed the processes of weathering and erosion to take place [17]. Researchers at UCLA studied 4.1 billion-year-old zircon crystals and found carbon in the zircon crystals that may be biogenic in origin, meaning that life may have existed on Earth much earlier than previously thought [18].

Table \(\PageIndex{2}\): Common isotopes used for radiometric dating [7; 8].

Elements	Parent symbol	Daughter symbol	Half-life
Uranium-238/Lead-206	^238U	^206Pb	4.5 billion years
Uranium-235/Lead-207	^235U	^207Pb	704 million years
Potassium-40/Argon-40	^40K	^40Ar	1.25 billion years
Rubidium-87/Strontium-87	^87Rb	^87Sr	48.8 billion years
Carbon-14/Nitrogen-14	^14C	^14N	5730 years

Limitations of Radiometric Dating

The principles behind this dating method require two key assumptions. First, the mineral grains containing the isotope formed at the same time as the rock, such as minerals in an igneous rock that crystallized from magma. Second, the mineral crystals remain a closed system, meaning they are not subsequently altered by elements moving in or out of them.

These requirements place some constraints on the kinds of rock suitable for dating, with the igneous rock being the best. Metamorphic rocks are crystalline, but the processes of metamorphism may reset the clock and derived ages may represent a smear of different metamorphic events rather than the age of original crystallization. Detrital sedimentary rocks contain clasts from separate parent rocks from unknown locations and derived ages are thus meaningless. However, sedimentary rocks with precipitated minerals, such as evaporites, may contain elements suitable for radiometric dating. Igneous pyroclastic layers and lava flows within a sedimentary sequence can be used to date the sequence. Cross-cutting igneous rocks and sills can be used to bracket the ages of affected, older sedimentary rocks. The resistant mineral zircon, found as clasts in many ancient sedimentary rocks, has been successfully used for establishing very old dates, including the age of Earth’s oldest known rocks [6]. Knowing that zircon minerals in metamorphosed sediments came from older rocks that are no longer available for study, scientists can date zircon to establish the age of the pre-metamorphic source rocks.

Figure \(\PageIndex{4}\): Granite (left) and gneiss (right). Dating a mineral within the granite would give the crystallization age of the rock while dating the gneiss might reflect the timing of metamorphism.

Ideally, several different radiometric techniques will be used to date the same rock. Agreement between these values indicates that the calculated age is accurate. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. For example, an overlying lava flow can give a reliable estimate of the age of a sedimentary rock formation in one location. Index fossils contained in this formation can then be matched to fossils in a different location, providing a good age measurement for that new rock formation as well. As this process has been repeated all over the world, estimates of rock and fossil ages have become more and more accurate.

Age of the Earth

The work of Hutton and other scientists gained attention after the Renaissance, spurring exploration into the idea of an ancient Earth. In the late 19^th century William Thompson, a.k.a. Lord Kelvin, applied his knowledge of physics to develop the assumption that the Earth started as a hot molten sphere. He estimated the Earth is 98 million years old, but because of uncertainties in his calculations stated the age as a range of between 20 and 400 million years [12; 13]. This animation illustrates how Kelvin calculated this range and why his numbers were so far off, which has to do with unequal heat transfer within the Earth. It has also been pointed out that Kelvin failed to consider pliability and convection in the Earth’s mantle as a heat transfer mechanism. Kelvin’s estimate for Earth’s age was considered plausible but not without challenge, and the discovery of radioactivity provided a more accurate method for determining ancient ages [14].

Figure \(\PageIndex{5}\): Artist’s impression of Earth in the Hadean Eon, early in Earth’s history.

In the 1950s, Clair Patterson (1922–1995) thought he could determine the age of the Earth using radioactive isotopes from meteorites, which he considered to be early solar system remnants that were present at the time Earth was forming. Patterson analyzed meteorite samples for uranium and lead using a mass spectrometer. He used the uranium/lead dating technique in determining the age of the Earth to be 4.55 billion years, give or take about 70 million (± 1.5%) [15]. The current estimate for the age of the Earth is 4.54 billion years, give or take 50 million (± 1.1%) [13]. It is remarkable that Patterson, who was still a graduate student at the University of Chicago, came up with a result that has been little altered in over 60 years, even as technology has improved dating methods.

Other Absolute Dating Techniques

Luminescence (aka Thermoluminescence): Radiometric dating is not the only way scientists determine numeric ages. Luminescence dating measures the time elapsed since some silicate minerals, such as coarse-sediments of silicate minerals, were last exposed to light or heat at the surface of Earth. All buried sediments are exposed to radiation from normal background radiation from the decay process described above. Some of these electrons get trapped in the crystal lattice of silicate minerals like quartz. When exposed at the surface, ultraviolet radiation and heat from the Sun release these electrons, but when the minerals are buried just a few inches below the surface, the electrons get trapped again. Samples of coarse sediments collected just a few feet below the surface are analyzed by stimulating them with light in a lab. This stimulation releases the trapped electrons as a photon of light which is called luminescence. The amount of luminescence released indicates how long the sediment has been buried. Luminescence dating is only useful for dating young sediment that is less than 1 million years old [20; 21]. In Utah, luminescence dating is used to determine when coarse-grained sediment layers were buried near a fault. This is one technique used to determine the recurrence interval of large earthquakes on faults like the Wasatch Fault that primarily cut coarse-grained material and lack buried organic soils for radiocarbon dating [22].

Figure \(\PageIndex{6}\): Thermoluminescence, a type of luminescence dating.

Fission Track: Fission track dating relies on damage to the crystal lattice produced when unstable ²³⁸U decays to the daughter product ²³⁴Th and releases an alpha particle. These two decay products move in opposite directions from each other through the crystal lattice leaving a visible track of damage. This is common in uranium-bearing mineral grains such as apatite. The tracks are large and can be visually counted under an optical microscope. The number of tracks corresponds to the age of the grains. Fission track dating works from about 100,000 to 2 billion (1 × 10⁵ to 2 × 10⁹) years ago. Fission track dating has also been used as a second clock to confirm dates obtained by other methods [23; 7].

Figure \(\PageIndex{7}\): Apatite from Mexico.

References

5. Geyh, M. A. & Schleicher, H. Absolute Age Determination: Physical and Chemical Dating Methods and Their Application, 503 pp. Spring-er-Verlag, New York (1990).

6. Wilde, S. A., Valley, J. W., Peck, W. H. & Graham, C. M. Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago. Nature 409, 175–178 (2001).

7. Dickin, A. P. Radiogenic isotope geology. (Cambridge University Press, 2005).

8. Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C. & others. Precision measurement of half-lives and specific activities of U 235 and U 238. Phys. Rev. C Nucl. Phys. (1971).

9. Dass, C. Basics of mass spectrometry. in Fundamentals of Contemporary Mass Spectrometry 1–14 (John Wiley & Sons, Inc., 2007).

10. Oberthür, T., Davis, D. W., Blenkinsop, T. G. & Höhndorf, A. Precise U–Pb mineral ages, Rb–Sr and Sm–Nd systematics for the Great Dyke, Zimbabwe—constraints on late Archean events in the Zimbabwe craton and Limpopo belt. Precambrian Res. 113, 293–305 (2002).

11. Burleigh, R. W. F. Libby and the development of radiocarbon dating. Antiquity 55, 96–98 (1981).

12. MacDougall, Doug. Nature’s clocks: how scientists measure the age of almost everything. (2008).

13. Brent Dalrymple, G. The Age of the Earth. (Stanford University Press, 1994).

14. Stacey, F. D. Kelvin’s age of the Earth paradox revisited. J. Geophys. Res. 105, 13155–13158 (2000).

15. Patterson, C. Age of meteorites and the earth. Geochim. Cosmochim. Acta 10, 230–237 (1956).

16. Ireland, T. New tools for isotopic analysis. Science 286, 2289–2290 (1999).

17. Valley, J. W., Peck, W. H., King, E. M. & Wilde, S. A. A cool early Earth. Geology 30, 351–354 (2002).

18. Bell, E. A., Boehnke, P., Harrison, T. M. & Mao, W. L. Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon. Proc. Natl. Acad. Sci. U. S. A. 112, 14518–14521 (2015).

19. Léost, I., Féraud, G., Blanc-Valleron, M. M. & Rouchy, J. M. First absolute dating of Miocene Langbeinite evaporites by 40Ar/39Ar laser step-heating:[K2Mg2 (SO4) 3] Stebnyk Mine (Carpathian Foredeep Basin). Geophys. Res. Lett. 28, 4347–4350 (2001).

20. Murray, A. S. & Olley, J. M. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16 (2002).

21. Murray, A. S. & Olley, J. M. Optically stimulated luminescence dating of sediments over the past 200,000 years. Annu. Rev. Earth Planet. Sci. 39, 461–488 (2011).

22. Christopher B. DuRoss, Stephen F. Personius, Anthony J. Crone, Susan S. Olig & William R. Lund. Integration of Paleoseismic Data from Multiple Sites to Develop an Objective Earthquake Chronology: Application to the Weber Segment of the Wasatch Fault Zone, Utah. Bulletin of the Seismological Society of America 101, 2765–2781 (2011).

23. P van den Haute & Frans De Corte. Advances in Fission-Track Geochronology. Springer Science & Business Media (2013).

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